A patent document 1 (JP-A-2005-265050) filed by the present applicant is proposing a technology related to a vehicle having a power transmission device in which a clutch and a transmission are coupled to the output shaft of an engine equipped with an exhaust turbocharger via a fluid coupling in order to connect/disconnect the transmission of power from the engine side to the transmission side by connecting/disconnecting the clutch, the fluid coupling incorporating a lockup clutch for directly coupling a pump and a turbine together.
In this power transmission device, a pump of the fluid coupling is coupled to the output shaft (crankshaft) of the engine equipped with the exhaust turbocharger, the turbine driven by the fluid torque produced by the pump is coupled to the input shaft of the clutch, and slippage between the pump and the turbine is utilized at the start of the vehicle to attain a smooth start. At the start, the clutch is connected, and the input shaft of the clutch coupled to the turbine of the fluid coupling is directly coupled to the input shaft of the transmission.
Upon attaining a preset low vehicle speed after the start of the vehicle, a lockup clutch control device connects the lockup clutch so as to couple the output shaft of the engine to the input shaft of the clutch in order to avoid loss that accompanies the transmission of power in the case of utilizing a fluid in the fluid coupling.
FIG. 5 is a diagram of lockup clutch control timings in an operation control device of a vehicle equipped with a lockup clutch for directly coupling a pump and a turbine together in a fluid coupling between an engine equipped with an exhaust turbocharger and a clutch such as one disclosed in the above patent document 1.
Referring to FIG. 5, after the operation via the fluid coupling for starting the vehicle, the rotational speed of the transmission input shaft (which is proportional to the vehicle speed and is equal to the rotational speed of the turbine of the fluid coupling) reaches a rotational speed holding the condition for connecting the lockup clutch (e.g., about 900 RPM), whereby the lockup clutch control device produces a lockup clutch connection instruction. Therefore, the lockup clutch is connected, and the output shaft of the engine is directly coupled to the input shaft of the clutch.
Patent document 1: JP-A-2005-265050